Various respiration detection and/or monitoring devices have been suggested and/or utilised in a variety of settings previously, and have included devices utilising impedance plethysmography, inductance plethysmography, aural monitoring, EMG or EKG monitoring, strain gauges or the like. These devices all have different limitations, including undue complexity for some uses, inability to monitor, or distinguish between, different types of respiratory and/or unrelated events such as upper airway obstructions, breath holding, sighing, yawning and artefact both of a mechanical and electromagnetic nature. Despite these limitations these devices are still used today as previous studies have established the crucial importance of respiratory monitoring for patient safety.
In recent years, pulse oximetry and capnography have been used to monitor heart rate and oxygen saturation. Pulse oximetry is described in U.S. Pat. No. 4,653,498 and utilises sonification. Although there have been experimental versions of sonifications designed to convey information about respiratory functioning (Fitch & Kramer, 1994; Loeb & Fitch, 2000) all have been in the context of a reworking of cardiovascular sonification and all have been experimental in nature. There has been no comparative study of the effectiveness of different respiratory sonifications under controlled conditions and no study of the effectiveness of respiratory sonification when other tasks must be performed, as is often the case in the operating theatre. Accordingly, there is no respiratory sonification in regular clinical use in health care or other physiological monitoring contexts.
The only devices used in the clinical context that provide auditory information about respiration are the ventilator and the precordial stethoscope. Ventilators are used in anaesthesia and intensive care contexts to support a patient's respiration, and usually deliver a base mixture of air and oxygen. In the anaesthesia context, a variety of anaesthetic gas can be added such as nitrous oxide, isoflurane, sevoflurane, halothane, and the like. Ventilators work in several modes, which can be roughly distinguished as follows:
1). spontaneous patient respiration;
2). manually assisted patient respiration; and
3). Machine-supported patient respiration.
Spontaneous patient respiration involves unassisted patient respiration, where respiration rate, airway pressure, tidal volume, volume flow, and end tidal carbon dioxide (ETCO2) can be measured. Manually assisted patient respiration involves patient respiration that is assisted by the anaesthetist who manually forces gas into the patient's lungs at regular intervals by squeezing a gas-filled bag attached to the ventilator.
Machine-supported patient respiration has a variety of modes relating to such things as aspects of the lung pressure that is maintained, the volume of gas that is delivered, the respiration rate maintained, the proportion of machine breaths that are given per patient breath if there is any patient respiration, and the ratio of the duration of inhalations to the duration of exhalations. Under machine ventilation, gas is automatically delivered to the patient through a mechanically-driven bellows.
In older anaesthesia machines and ventilators, the bellows was in plain view, and its operation could be reasonably clearly heard alongside other ambient noise. However, in newer anaesthesia machines and ventilators, there has been a trend to make the bellows, operation quieter and to place the bellows out of sight. Therefore the informal information provided about machine-supported patient respiration that an older generation of ventilators provided is starting to disappear, with possible adverse consequences.
The precordial stethoscope incorporates a sensor affixed to the patient's chest, connecting tubing as seen in a normal stethoscope, and typically a single ear piece worn by the anaesthetist or critical care provider. The precordial stethoscope amplifies and, delivers to the ear high-fidelity, naturally generated sound generated by the heart and the lungs. Not only can respiration rate and the ratio of inhalation to exhalation be heard and depth of respiration inferred, but also a wide variety of lung sounds and qualities that suggest inappropriate intubation (positioning of a breathing tube), blockages and occlusions, respiratory abnormalities and lung diseases. However there are disadvantages to the precordial stethoscope as a means of continuously monitoring patient respiratory functioning. For example, the precordial stethoscope provides no information about ETCO2, which relies upon the presence of capnometry; the sensor and ear piece need to be continuously in place for monitoring to take place, which may not always be convenient.
Accordingly, neither the ventilator nor the precordial stethoscope provides a satisfactory means of monitoring respiration in a subject. Therefore, there is a real need for a means of monitor human respiration, especially during anaesthesia and intensive care.
The applicant has now developed a respiratory sonification means and method that overcomes or at least alleviates some of the problems highlighted above.